(1) Field of the Invention
The present invention relates to an interpolation apparatus that generates interpolation pixel values necessary for converting input video data of interlace scanning into video data of progressive scanning, and to a video signal processing apparatus including the interpolation apparatus. In particular, the present invention relates to improvements in interpolation processing.
(2) Related Art
Scan line conversion techniques for converting an input video signal of interlace scanning into a video signal of progressive scanning can be roughly categorized into two types: xe2x80x9cinter-field interpolationxe2x80x9d and xe2x80x9cintra-field interpolationxe2x80x9d. Inter-field interpolation generates an interpolation scan line using a video signal of a preceding field, whereas intra-field interpolation generates an interpolation scan line using a video signal within a present field. Intra-field interpolation is employed more frequently due to its simple interpolation processing.
An I/P conversion circuit, one example of which is shown in FIG. 1, is conventionally known as a technique to realize intra-field interpolation.
As the figure shows, the I/P conversion circuit 110 includes an input terminal 100, a line memory 101, a pixel difference detection circuit 102, a correlation direction detection circuit 103, an interpolation pixel generation circuit 104, and a time axis conversion circuit 105.
An interlaced video signal (X1) is inputted into the input terminal 100. The input video signal (X1) is transmitted to the line memory 101, the pixel difference detection circuit 102, and the interpolation pixel generation circuit 104.
The line memory 101 delays the video signal (X1) transmitted from the input terminal 100 for a time period corresponding to one scan line (1-line), and outputs the delayed video signal as a 1-line delay signal (X2), to the pixel difference detection circuit 102 and the time axis conversion circuit 105.
The processing described above enables adjacent two lines within one field of the interlaced video signal to be inputted into the pixel difference detection circuit 102.
The pixel difference detection circuit 102 selects, from pixels on the adjacent two lines, a plurality of pixel pairs each including two pixels that are respectively on the adjacent two lines and that are symmetric with respect to a position of a pixel that is going to be interpolated (hereafter, a xe2x80x9cpixel that is going to be interpolatedxe2x80x9d is referred to as an xe2x80x9cinterpolation pixelxe2x80x9d). The pixel difference detection circuit 102 calculates a difference in luminance between two pixels (hereafter referred to as a xe2x80x9cluminance differencexe2x80x9d) in each selected pixel pair. The pixel difference detection circuit 102 then outputs each calculated luminance difference as a pixel difference detection signal (X3) for the interpolation pixel.
The correlation direction detection circuit 103 selects a pixel pair with the smallest luminance difference, using the pixel difference detection signal outputted from the pixel difference detection circuit 102. The correlation direction detection circuit 103 then detects a direction of a straight line that links the two pixels in the selected pair, and outputs a signal indicating the detected direction as a correlation direction signal (X4).
The interpolation pixel generation circuit 104 determines the two pixels that are respectively on the two lines and that are in the direction with the smallest luminance difference, using the video signal (X1), the 1-line delay signal (X2), and the correlation direction signal (X4). The interpolation pixel generation circuit 104 averages the luminance of the determined two pixels, and sets the averaged value as an interpolation value for the interpolation pixel.
The interpolation value being generated by averaging the luminance of the two pixels positioned in such a direction that has the smallest luminance difference is due to the following reason.
A sequence of pixels with similar luminance is most likely to extend in the direction of the straight line that links the two pixels with the smallest luminance difference. Being positioned on the straight line that links the two pixels, i.e., positioned on the sequence of the pixels with similar luminance, the interpolation pixel must have the highest correlation with the two pixels with the smallest luminance difference. Therefore, it is considered most appropriate to generate an interpolation value based on the luminance of these two pixels. The direction in which a pixel pair with the smallest luminance difference is positioned with respect to the interpolation pixel is hereafter referred to as the xe2x80x9ccorrelation directionxe2x80x9d.
The pixel difference detection circuit 102, the correlation direction detection circuit 103, and the interpolation pixel generation circuit 104 sequentially execute the above described processing on each pixel to be interpolated, and outputs an interpolation signal (X5) that indicates the generated interpolation values of the interpolation pixels.
The time axis conversion circuit 105 receives the 1-line delay signal (X2) and the interpolation signal (X5), and sequentially subjects the 1-line delay signal (X2) and the interpolation signal (X5) to the time compressed integration process, to output a progressive scanned video signal (X6).
The processing describe above enables the interpolation signal (X5) to be generated using the video signal (X1) and the 1-line delay signal (X5), and the interpolation line to be inserted at the interpolation line position.
With the interpolation described above, an interpolation pixel can be generated based on pixels positioned in such a direction that has the highest correlation with the interpolation pixel, thereby improving an image quality, compared with when an interpolation pixel is generated based on pixels that are not correlated with the interpolation pixel.
This interpolation, however, may be flawed because a direction with the smallest luminance difference is always set as the correlation direction. The problem may arise, for example, when the luminance of one pixel in the direction detected as the correlation direction with the smallest luminance difference is being influenced by noise. In this case, a completely wrong direction may be set as the correlation direction. If this happens, an interpolation pixel is generated based on pixels that are not correlated with the interpolation pixel, resulting in the interpolation contrarily deteriorating the image quality.
Further, in the case of a still image area, an interpolation line can be actually reconstructed using an input video signal of a field preceding the present field that includes interpolation pixels. Despite this fact, however, the above interpolation has conventionally been performed regardless of whether an interpolation pixel is in a still image area or in a moving image area with high motion. This creates unnecessary possibility of deteriorating the image quality due to the interpolation performed with being influenced by noise, even when an interpolation pixel is in a still image area.
In view of the above problem, a first object of the present invention is to provide an interpolation apparatus that can minimize image quality deterioration caused by noise influence on interpolation.
A second object of the present invention is to provide a video signal processing apparatus that can minimize image quality deterioration caused by noise influence on interpolation in a moving image area, and can nearly eliminate image quality deterioration caused by noise influence on interpolation in a still image area, by incorporating the interpolation apparatus therein.
The first object of the present invention can be achieved by an interpolation apparatus that generates interpolation pixel values necessary for converting input video data of interlace scanning into video data of progressive scanning, the interpolation apparatus including: a selection unit for selecting, from pixels on adjacent two scan lines within one field of input video data, a plurality of candidate pixel-pairs, each of which is composed of two pixels that are symmetric with respect to a position of a pixel that is going to be interpolated; a calculation unit for calculating a difference between pixel values of each selected candidate pixel-pair; and a generation unit for (a) determining, from the selected candidate pixel-pairs, a pixel-pair to be used for generating an interpolation pixel value of the pixel that is going to be interpolated, based on a smallest difference and a 2nd smallest difference of the calculated differences, and (b) generating the interpolation pixel value, based on pixel values of the determined pixel-pair.
With this construction, an interpolation value is generated based on the difference between the smallest difference and the 2nd smallest difference. By setting in advance such ranges of the smallest difference and the 2nd smallest difference where pixel values of pixels positioned in a direction with the smallest difference are highly likely to have been influenced by noise, interpolation based on the pixels in the direction with the smallest difference can be prevented when the smallest difference and the 2nd smallest difference are respectively in the set ranges. Accordingly, noise influence on interpolation can be decreased, thereby decreasing the image quality deterioration.
The second object of the present invention can be achieved by a video signal processing apparatus that converts input video data of interlace scanning into video data of progressive scanning, the video signal processing apparatus including: a selection unit for selecting, from pixels on adjacent two scan lines within one field of input video data, a plurality of candidate pixel-pairs, each of which is composed of two pixels that are symmetric with respect to a position of a pixel that is going to be interpolated; a calculation unit for calculating a difference between pixel values of each selected candidate pixel-pair; a first generation unit for (a) determining, from the selected candidate pixel-pairs, a pixel-pair to be used for generating an interpolation pixel value of the pixel that is going to be interpolated, based on a smallest difference and a 2nd smallest difference of the calculated differences, and (b) generating the interpolation pixel value, based on pixel values of the determined pixel-pair; a second generation unit for generating an interpolation pixel value of the pixel that is going to be interpolated, by referring to a pixel value of a pixel that corresponds to the pixel to be interpolated and that is in a field immediately preceding a present field to which the pixel to be interpolated belongs; a detection unit for detecting a change in an image of the present field, by referring to the field immediately preceding the present field and a field immediately following the present field; a selection unit for selecting, according to a detection result by the detection unit, one of (c) the interpolation pixel value generated by the first generation unit and (d) the interpolation pixel value generated by the second generation unit; and an output unit for alternately outputting (e) scan lines interpolated using interpolation pixel values selected by the selection unit and (f) scan lines of the input video data.
With this construction, one of (a) the pixel value generated by the first generation unit and (b) the pixel value generated by the second generation unit can be selected for use in interpolation, in accordance with a change in an image. For example, when the interpolation pixel is in a still image area, the pixel value generated by the second generation unit is selected for use in interpolation. This enables interpolation to be performed without noise influence in a still image area. Therefore, an image quality can be improved.